As a general rule, ballasts can only be used with bulbs that are of the same wattage rating. A 1000W ballast can only be used with a 1000W bulb, a 400W with a 400W, etc. The one exception is the Xtrasun "Dial-a-Watt" ballasts (XTEDW-), which are meant to be used with the same wattage bulb as the dimmed setting (for example, use a 600W bulb when dimmed to 600W, etc.).

Can an HPS ballast be used to power a metal halide bulb?

This is never recommended, the ballast and bulb type (wattage and HID bulb type) must match. Convertible ballasts, however, can be switched between MH and HPS modes.
There is also the option of using a conversion bulb. This is a bulb that is designed to produce a MH spectrum on a HPS ballasts, or vice versa.
Phantom digital ballasts can ignite HPS or MH bulbs, except for the 750W model, which is meant only for HPS.